A roof structure often is comprised of a water-impermeable roofing layer disposed over thermal insulation panels supported by a roof deck which may consist of a corrugated steel deck supported by structural I-beams. To secure the water-impermeable roofing layer and thermal insulation panels to the roof deck, fasteners are driven or screwed or otherwise forced through the roofing layer and insulation panels and through the roof deck. The fastener may be associated with a metal plate by threading the fastener through a hole in the metal plate before the fastener is driven or screwed into place in the roof structure. The metal plate is intended to aid in preventing the head of the fastener from being forced through the water-impermeable roofing layer and into the insulation panel. Generally, the conventional metal plate is formed of relatively thin metal sheet material. The metal plate also is intended to hold the roofing layer and insulation panel firmly against the roof deck during those periods when winds are blowing across the roof structure causing an uplift force tending to pull the roofing layer and insulation panels free from the roof deck. The metal plates being relatively thin are subject to edge-bending during installation. When walked upon or when wind uplift forces are created after the roof has been installed, the relatively sharp bent edges of the metal plates often penetrate through the thin roofing layer and destroy the water-impermeable integrity of the roof structure.
A plastic fastener plate intended to overcome many of the deficiencies inherent in the metal plates heretofore used in roof structures is described in U.S. Pat. No. 4,361,997, the disclosure thereof being incorporated herein by reference. In accordance with the disclosure in the aforesaid patent, the plastic plate is comprised of a substantially planar section that has a hub segment depending from the planar section and anti-rotational ribs or splines or the like which are forced into the insulation panels to prevent rotation of the plastic plate after installation. A hole extends through the hub and a counterbore is provided in the planar section to allow a fastener to be assembled with the plastic plate.
The above-described plastic plates cannot be conveniently used when a water-impermeable roofing layer is disposed over the insulation panels since the depending hub segment and ribs or splines cannot easily penetrate into the insulation panels, but, instead would ride on top of the roofing layer. Penetration of the depending hub and ribs or splines into insulation panels that have a relatively thick "skin" surface also would be difficult. Furthermore, even when the hub and ribs or splines of the plastic plate can be forced into the insulation panels as described in the aforementioned patent, difficulty sometimes is experienced when the fastener is driven through the roof deck if too great a driving force is used, since the head of the fastener can cause splitting of the hub segment. Also, when a wind of substantial velocity blows across a roof structure in which such aforesaid plastic plate and fastener assemblies are used, the wind uplift forces are concentrated at the location of the hub segment and tend to cause the fastener head to split the hub segment.